This invention generally relates to laser printers and more particularly, this invention relates to a laser printer having selectable printing resolutions.
Within the last decade, laser printers have become the standard against which all other printers are measured in the personal computing industry. Understandably, a large amount of research has been recently undertaken to increase their versatility and print quality. This has resulted in the development of personal laser printers having resolutions in excess of 1200 DPI (dots-per-inch).
FIG. 1 of the attached drawings shows a representational diagram of a basic laser printer configuration, to which the reader is referred to aid in the following explanation of the electrophotographic process for recording and registering an image on paper. In general terms, a computer, or similar device, sends a series of codes representing an image to the input port 27 of the laser printer. The laser printer converts the codes to a series of binary signals, each of which generally represents one dot of the thousands of dots which together form the image. The binary signals are used to pulse the beam of a laser such that the binary pattern is represented or transmitted by turning off and on the laser. This pattern is then recorded on a light sensitive drum which transfers the pattern onto a printing medium, such as paper, in the form of toner or a similar indelible substance. In general terms, an image forming device of this nature has two main components, a raster image processor for processing the image and a print engine to fix the image to media.
As can be seen in FIG. 1, once the data has been transmitted to the laser printer it is analyzed in Formatter 11, here consisting of a microprocessor 23 and related memory 24 and buffer 12. Formatter 11 parses out the printer control commands from the text and graphics, and manipulates the text and graphics in accordance with the printer control commands and user input control codes from user input and display panel 25. Once the page has been formatted, it is transmitted to the buffer 12. Data is then sent to a circuit which drives the laser 13.
The data is used to modulate a light beam produced by laser 13. The modulation of the beam is accomplished by laser driver-controller 26. The laser beam is then reflected off of a multifaceted, spinning mirror 14. Here, the multifaceted mirror is shown as a six sided polyhedron, however as few as two facets are common. As each facet of the mirror 14 spins through the light beam, it reflects, or xe2x80x9cscansxe2x80x9d, the beam across the side of a photoconductive drum 15. The photoconductive drum 15 is rotated about its axis such that it advances just enough that each successive scan of the light beam is recorded immediately after the previous scan directly on the photoconductive drum 15. In this manner, each strip of data from the buffer 12 is recorded on the photoconductive drum 15, as a line one after the other to reproduce the page on the drum.
The laser beam actually discharges the area on the photoconductive drum 15 it irradiates. The photoconductive drum 15 is first charged using a high voltage primary corona wire, shown at 16, to have a negative polarity at its surface. Because of the special photoconductive material which covers the drum, the laser beam effectively discharges any areas which it irradiates. This process creates a xe2x80x9clatentxe2x80x9d electrostatic image on the drum. This portion of the drum then comes into close proximity to the developing roller 17 which rotates counter clockwise, or opposite to the photoconductive drum 15. The developing roller 17 transfers the toner from the toner bath to the photoconductive drum 15. Here, a dry toner is used which consists of fine thermoplastic particles impregnated with a ferromagnetic material such as iron. The developer roller 17 uses the negative pole of an internal magnet to attract the toner. Triboelectric charging results in a negative charge to the particles themselves. The developer roller 17 is electrically biased so as to repel the charged toner to the image areas. In this manner, the toner is transferred to the photoconductive drum 15 and forms a pattern thereon which duplicates the image.
The toner is transferred from the photoconductive drum 15 to the printing medium, e g. paper 18, using an electrostatic process. Media drive system 28 advances the sheet of media through the printer. Here a second corona wire, transfer corona 19, is used to impart a relatively strong positive charge to the back side of the paper 18 as it passes by the photoconductive drum 15. The high positive charge attracts the negatively charged toner and pulls it from the drum, maintaining the same pattern. The toner is then fused to the paper 18 by passing both the toner and paper through a pair of hot fusing rollers 20.
The photoconductive drum 15 usually has a circumference which is less than the length of most paper. Hence, the drum must rotate several times to print a full sheet. The drum is cleaned with cleaning blade 21, completely discharged by discharge lamps 22 and recharged by corona 16.
The following discussion concerns references, of which the inventors are aware, showing technologies and improvements related to the instant invention. U.S. Pat. No. 4,578,689 issued March, 1986 to SPENCER ET AL. for a Dual Mode Laser Printer teaches a laser printer having two modes for printing at different resolutions. The first is a high speed/low resolution mode and the second is a low speed/high resolution mode. The disclosure claims that the printer is capable of printing up to 16 pages per minute in low resolution mode, i.e. less than 400 DPI (dots-per-inch), and at approximately 4 pages per minute in high resolution mode, i.e. above 500 DPI. The printer accomplishes this by changing the speed of the paper drive and photoconductive drum using stepper motors, adjusting the heat produced by the fuser, adjusting the corona current in the electrophotographic process, controlling the laser output power and controlling the number of facets of the rotating mirror which are actually used to scan the photoconductive drum. The speed of the rotating mirror is kept constant regardless of the resolution selected. When reducing both the speed of the paper and the photoconductive drum, only every other, or fewer, facets of the rotating mirror are used, thus allowing enough time to elapse to permit the information to be accumulated without increasing the data transfer rate to the printer.
U.S. Pat. No. 4,700,201 issued October, 1987 to SATO for a Dot Corrected Laser teaches an image enhancement technique which varies the size of the dot produced dependant upon whether the dot is relatively isolated, e.g. not completely surrounded by other dots immediately adjacent to it, or in a densely populated area of the image. If a particular dot is determined to be isolated, the invention will increase the size of the dot by varying the duty cycle of the laser. In a positive exposure system, the xe2x80x9con timexe2x80x9d of the laser is increased while in a negative exposure system the xe2x80x9coff timexe2x80x9d of the laser is increased. The invention uses either a modified driver circuit or an acousto-optic modulator to control the duty cycle of the irradiation to vary the spot or dot sized produced.
U.S. Pat. No. 4,717,925 issued January, 1988 to SHIBATA ET AL. for an Optical Scanner Without Extra Convergent Lens teaches a scanner error correction device which is similar in concept to that described in U.S. Pat. No. 4,613,877 discussed above. Additionally, this patent teaches an adjustable intensity laser beam using a photodetector, cooperating driver controller and laser driver to continuously adjust the intensity of the beam. As the data video rate increases, the intensity of the beam is increased to provide a uniform spot or dot size regardless of the shortened duty cycle.
U.S. Pat. No. 4,734,715 issued March, 1988 to SHIRAISHI for a Variable Light Beam Scanning Apparatus teaches a variable resolution laser printer. The variable resolution is accomplished by controlling the scan velocity of the laser beam, the video data frequency at which the laser beam is modulated and the spot diameter or dot size. The scan velocity of the laser beam is varied by controlling the rotational speed of the polyhedron faceted mirror, while the video data rate or frequency is varied by adjusting the clock rate and the laser spot diameter is varied by controlling the drive current supplied to the laser diode.
U.S. Pat. No. 4,742,363 issued May, 1988 to SHIRAISHI for a Variable Intensity Light Beam Scanning Apparatus With Feedback teaches essentially the same laser printer disclosed in U.S. Pat. No. 4,734,715 but concentrates on the circuit for controlling the light intensity of the laser beam produced by the laser diode by controlling the drive current.
U.S. Pat. No. 4,899,176 issued February, 1990 to MCQUADE for a Method of Reducing Average Data Rate in Rotating Mirror Laser Recorder teaches a method and apparatus for matching the rate at which the laser printer processes data to the rate at which the host device, such as a personal computer, supplies data to be printed. This is accomplished by reducing the number of facets of the polyhedron mirror which are actually used to scan data lines and also reducing the paper travel rate through the electrophotographic process.
U.S. Pat. No. 4,953,036 issued August, 1990 to YOSHIMURA for a Digital Copier Having Switchable Pixel Densities teaches a digital copy machine/laser printer having a high resolution mode (in excess of 400 DPI) for digital copy reproduction and a low resolution mode (240/300 DPI) for printing as a laser printer. The resolution switching is provided by one or more of the following configurations including: two separate lens systems, one for producing a reduced dot size and one for the regular dot size; a single lens system positionable at different points in the optical path of the laser to vary the spot size; changing the illumination time of the laser beam; or changing the rotational speed of the polyhedron mirror.
U.S. Pat. No. 5,072,303 issued December, 1991 to SILVERBERG teaches a laser printer which acts both as a standard 300 DPI laser printer and as a device for receiving and printing fax images. The printer is capable of printing in a 300xc3x97300 DPI laser printer mode, a 200xc3x97200 DPI xe2x80x9cfinexe2x80x9d fax mode and a 100xc3x97200 xe2x80x9cnormalxe2x80x9d fax mode. The patent further teaches controlling the spot size produced by the laser diode using pulse width modulation of the drive signal or by controlling the drive current supplied to the laser light source.
Unfortunately, all of the prior art solutions of which the inventors are aware require alteration of almost the entire image forming device system including the paper path speed, laser scanner, fuser, supporting electronics and firmware. Additionally, because of the focus on increasing resolution, print engines are currently designed around higher and higher resolutions, resulting in xe2x80x9cdraft modesxe2x80x9d with unnecessarily high resolutions which take longer to print and use more toner than necessary.
One embodiment of the invention has an image forming device including a fast draft mode accomplished by speeding up the media through the image forming device and scaling the image data in hardware, separate from the firmware. Optionally, the image can also be enhanced to improve image quality.
By speeding up the media path, commonly referred to as the paper path, and not the scanner, fewer print lines are needed per page. Because fewer lines are printed per page of data the aspect ratio must be altered to maintain a full page printout. A scaling apparatus is used to shrink the electronic image vertically by the inverse of the amount the speed has been increased resulting in an image having the same aspect ratio as the original. By implementing the scaling apparatus in hardware inside the print path and outside of the firmware, the firmware only has to deal with the image in its original size. The image source is independent of the media path speedup. Additionally, the scaling apparatus can be programmed to scale the image to an appropriate size to match any engine speedup, thereby allowing the design to be easily matched to whatever speedup amount a particular engine design can attain. For any particular print engine the amount of output path speedup is limited by the physical limits of the motor, driver and fuser as well as the amount of output quality degradation that is acceptable in draft mode.
Another embodiment of the invention provides image enhancement of draft mode images. A first order enhancement uses a filter to add dots below and to the right of any existing dot to produce a printed image which is nearly as bold as the original image. Another embodiment adds dots above and to the left of any existing dot. If sub-pixel modulation is available, an additional order of enhancement can be accomplished by a second filter to smooth edges and attempt to fill and correct gaps between the lines. Another embodiment increases the boldness of the printed image by increasing the power to the laser to increase its density setting. Another embodiment adjusts the developer bias to increase the density. Another embodiment optimizes templates and filters for each printing speed. Another embodiment utilizes a multi-bit scaler in place of the one bit scaler to produce additional information about the scaled dots which can be used to further enhance the image. Other embodiments use combinations of the previously mentioned embodiments.